In dialysate preparation apparatus of the type shown in Johnson U.S. Pat. No. 4,371,385 (which is hereby incorporated by reference), water is heated, deaerated, and mixed with dialysate concentrate to continuously provide dialysate supplied to a dialyzer used with a patient. In such apparatus, dialysate concentrate is typically pumped to a junction on a main flow line for mixing with water. A downstream conductivity cell is used to sense the conductivity of the mixed solution passing through it (which conductivity is related to the concentration of elements of the dialysate in the water), and this is used in a control loop to control the pump for dialysate concentrate. When such apparatus is disinfected and cleaned, between uses with different patients, bleach or disinfectant is flushed through the hydraulic circuitry of the dialysate preparation apparatus. "Cleaning fluid" herein means bleach or disinfectant or other chemicals which are flushed through the hydraulic circuitry.